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Mineralogy and Petrology of Yamato 000593: Comparison with Other Martian Nakhlite Meteorites

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Mineralogy and Petrology of Yamato 000593: Comparison with Other Martian Nakhlite Meteorites Antarct. Meteorite Res., +0, -.῍/1, ,**- ῌ National Institute of Polar Research Mineralogy and petrology of Yamato ***/3-: Comparison with other Martian nakhlite meteorites Takashi Mikouchi, Eisuke Koizumi, Akira Monkawa, Yuji Ueda and Masamichi Miyamoto Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Hongo 1-chome, Bunkyo-ku, Tokyo ++-῍**-- Abstract: Yamato (Y) ***/3- is a new nakhlite recovered from Antarctica and is composed of roughly 2*ῌ augite, +*ῌ olivine and +*ῌ mesostasis. Augite is chemically homogeneous except for Fe-rich rims adjacent to the mesostasis. Olivine has more extensive chemical zoning, but the most Fe-rich part is also near the mesostasis. These observations suggest that chemical zoning of both augite and olivine was produced by interaction with the mesostasis. The crystallization history of Y***/3- as deduced from this study is as follows. (+) Crystallization of cumulus augite and olivine and formation of symplectites in olivine. (,) Accumulation of augite and olivine. (-) Mesostasis crystallization and interaction of the augite and olivine rims with the intercumulus melt. (.) Aqueous alteration. The petrography and mineralogy of Y***/3- is generally similar to other nakhlites, but minor mineral- ogical di#erences are observed. These di#erences resulted from di#erent thermal histories due to di#erent locations (burial depths) in the same cooling cumulate pile. Y***/3- is most similar to Nakhla and both samples experienced similar formation histories. However, later mesostasis crystallization of Y***/3- was more rapid than Nakhla due to its faster cooling rate. The burial depth of Y***/3- would be shallower than - m from the surface, and is intermediate between NWA2+1 and Nakhla. The abundance and mineralogy of the mesostasis as well as augite and olivine rim compositions are related to the burial depths of nakhlites. key words: nakhlite, augite, olivine, crystallization, cooling rate +. Introduction Among more than twenty known samples of Martian meteorites, six samples (Nakhla, Lafayette, Governador Valadares, Northwest Africa 2+1, Yamato ***/3-/ ***1.3/***2*, and Northwest Africa 332) show similar petrography and chemistry, constituting a “nakhlite” group( e.g., Bunch and Reid, +31/; Harvey and McSween, +33,a; Imae et al., ,**,a; Irving et al., ,**,; McSween, +33.; Meyer, +332; Sautter et al., ,**,). Nakhlites are cumulus clinopyroxenites with minor amounts of olivine and mesostasis. All the known nakhlites show very similar mineralogy, as well as similar crystallization and cosmic-ray exposure ages (+.- Ga and +*῍++ Ma, respectively) (e.g., Bogard et al., +32.; Bogard, +33/; Bunch and Reid, +31/; Eugster et al., +331; Harvey and McSween, +33,a; Marty et al., ,**+; Mikouchi and Miyamoto, +332, ,**,; Nakamura et al., +32,; Reid and Bunch, +31/; Sautter et al., ,**,; Shih et al., +330, -. Mineralogy and petrology of the Y***/3- nakhlite 35 +332; Wadhwa and Crozaz, +33/). However, minor di#erences in mineralogy have been pointed out among nakhlite samples (e.g., Harvey and McSween, +33,a; Lentz et al., +333; Mikouchi and Miyamoto, +332, ,**,; Sautter et al., ,**,). It is believed that such mineralogical di#erences, albeit slight, are due to di#erent degrees of late magmatic and subsolidus atomic di#usion due to di#erent locations in the same cooling cumulate pile (e.g., Harvey and McSween, +33,a). Therefore, the discovery of new nakhlite samples could o#er substantial information to better understand the formation condi- tions and geological setting of this unique Martian meteorite group. Yamato ***/3-, Yamato ***1.3, and Yamato ***2*,, which are possibly paired, are the first samples of nakhlites from Antarctica (Kojima and Imae, ,**+; Imae et al., ,**,a, b). Yamato ***/3- (hereafter, Y***/3-) is the largest achondrite sample in the Antarctic meteorite collections (Imae et al., ,**,c). In this paper, as a part of the consortium study of the Yamato nakhlites (Kojima et al., ,**,), we present mineralogy and petrology of Y***/3- to understand its formation conditions. We also compare the mineralogy of Y***/3- with those of other nakhlites to discuss the mineralogical relationships between Y***/3- and other nakhlites. ,. Samples and analytical techniques We studied three polished thin sections of Y***/3- (Y***/3-,0,--,Y***/3-,0,-0, and Y***/3-,02-+) supplied from the National Institute of Polar Research (NIPR). Back-scattered electron (BSE) images were taken with JEOL JXA2.* and Hitachi S- ./** (field emission gun) scanning electron microscopes with energy dispersive spec- trometers (EDS), respectively (Dept. of Earth and Planet. Sci., University of Tokyo). X-ray elemental distribution maps were acquired by a JEOL JXA 23**Lelectron microprobe (Dept. of Earth and Planet. Sci., University of Tokyo). Accelerating voltage was +/ kV, and the beam current was 0*ῌ2* nA. Quantitative wavelength dispersive analyses were performed on a JEOL Superprobe 1-- electron microprobe (Ocean Research Institute, University of Tokyo) and a JEOL JCM 1-- mk II micro- probe (Dept. of Earth and Planet. Sci., University of Tokyo) by using natural and synthetic standards. Microprobe analyses of most phases were obtained by focused beam at +/ kV accelerating voltage with a beam current of +, nA. For feldspar minerals in the mesostasis, a defocused beam of ῌ/ mm diameter was employed. The petrography and mineral compositions of Y***/3- were compared with those of other nakhlites by using data contained in Mikouchi and Miyamoto (+332) and Mikouchi and Miyamoto (,**,) for Nakhla, Governador Valadares and Lafayette and Mikouchi and Miyamoto (,**+) for Northwest Africa 2+1 (hereafter, NWA2+1). Because we have not analyzed Northwest Africa 332 and its available mineralogical data are limited at present, we do not consider this meteorite in this paper. -. Petrography All the Y***/3- thin sections studied show a similar unbrecciated cumulus texture with the modal abundances of minerals, ῌ2*ῌ augite, +*ῌ olivine, and +*ῌ mesostasis (Fig. +). We did not observe any obvious di#erences in modal abundances among the 36 T. Mikouchi et al. Fig. +a. Optical photomicrograph (open nikol) of the Y***/3- thin section (Y***/3-,02-+). The thin section shows a cumulate texture mainly composed of augite, oli- vine and the mesostasis. Augite shows a prismatic crystal texture up to +./ mm long. Fig. +b. Back-scattered electron (BSE) image of the same field of view as Fig +a. Bright phases are mostly olivines. Augite is princi- pally homogeneous except for the rims adjacent to the mesostasis. Aug: augite. Ol: olivine. Meso: mesostasis. three thin sections. Augite commonly occurs as euhedral to subhedral elongated grains whose longer dimension reaches up to +./ mm (Fig. +). Polysynthetic twinning is commonly ob- served. It is likely that augite is a cumulus phase as in other nakhlites, because of its high abundance and a preferred orientation (though not so obvious in our samples) of the longer dimensions of the grains. Minor amounts of low-Ca pyroxene are present (ῌ+ volῌ) and are usually associated with the mesostasis. Olivine grains are mostly anhedral and interstitial to cumulus augite (Fig. +b). The size of typical olivine grains is similar to that of pyroxene and they sometimes show euhedral crystal termination adjacent to the mesostasis. Small anhedral olivine grains (less than ,** mm in size) are also found and they are all interstitial to augite. Dark lamellar or patchy inclusions are commonly observed in large olivine grains (Fig. ,a). Mineralogy and petrology of the Y***/3- nakhlite 37 Fig. ,a. Optical photomicrograph (open nikol) of one of the largest olivine grains in Y***/3-.The presence of dark lamellar (or rectangular in this image) inclu- sions throughout the crystal is re- markable. Brown alteration prod- ucts are observed around the rim and the fractures. Ol:oli- vine. Fig. ,b. Optical photomicrograph (open nikol) of an altered olivine grain in Y***/3-. The presence of brown alteration products along the fractures is remarkable in this olivine grain. This olivine grain contains a poikilitic augite grain (Aug-p). The mesostasis (Meso) is also showing alteration similar to olivine. Ol:olivine. Aug:augite. Fig. ,c Optical photomicrograph (open nikol) of the mesostasis area in Y***/3-. Note the pres- ·-� ence of brown alteration prod- ucts interstitial to plagioclase laths. The mesostasis includes a Ti-rich magnetite grain (Ti-mag). Some of the tiny opaque grains in the mesostasis are Fe sulfide. : 100 µm Aug:augite. Ol:olivine. 38 T. Mikouchi et al. The distributions of these lamellar inclusions within olivine grains are irregular unlike regular exsolution lamellae. Some large olivine grains poikilitically enclose rounded augite grains whose composition is identical to cumulus augite (Fig. ,b). Although magmatic inclusions are uncommon in olivine grains, single crystal augite inclusions, similar to those reported in Harvey and McSween (+33,b), are observed in some olivine grains. These inclusions are rounded with sizes up to ,** mm and are sometimes associated with K-feldspar. Olivine grains are pervasively altered to brown-colored, fine-grained materials (smectite ?) along fractures (Fig. ,b) as reported in Imae et al. (,**,b) and Treiman and Goodrich (,**,). The mesostasis commonly displays a plumose texture, which is composed pre- dominantly of narrow laths of plagioclase with subordinate proportions of K-rich feldspar and a silica mineral (Fig. ,c). Plagioclase length reaches up to ,**ῌ-** mm although
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